The experiments described in this proposal address one of the most fascinating unanswered questions in vision research regarding the molecular mechanisms responsible for photoreceptor outer segment morphogenesis. The outer segment is a ciliary organelle that produces electrical signals in response to capturing light. A unique morphological feature of the outer segment is that it is filled with a stack of flattened membrane discs providing vast surfaces for photon capture and signal amplification. The functional significance of this anatomical arrangement has been recognized for a very long time, yet our understanding of how discs are built at the molecular level remains frustratingly rudimentary. This application addresses several poorly understood aspects of photoreceptor disc morphogenesis, related to the processes of disc expansion, alignment and enclosure. Our preliminary data show that the edges of newly formed discs contain two distinct types of extracellular links: one connecting discs with the inner segment plasma membrane and another connecting disc edges between themselves. Experiments described in Aims 1 and 2 will be devoted to determining the protein composition of each link type and elucidating their specific roles in supporting the high fidelity of disc elongation and stacking. Aim 3 will focus on the final step in disc maturation consisting of its scission from the outer segment plasma membrane. We will address whether disc scission takes place exclusively in rods and explore molecular players involved in this process. Experiments described in this application will employ versatile molecular tools combined with the state-of-the-art three dimensional electron microscopy tomographic analysis of the outer segment structure. Addressing these mechanistic questions is essential for advancing our basic understanding of photoreceptor cell biology, as well as elucidating the pathophysiological mechanisms underlying inherited blindness frequently associated with defects in outer segment morphogenesis.